1. Field of Invention
The invention relates to an automatic document feeder used in scanners, copiers, fax machines, or multifunction processors. It is used to separate paper in a pile and to feed it sheet by sheet into a machine for further processing.
2. Related Art
FIG. 1 shows a conventional paper picking/feeding mechanism used in fax machines. It mainly comprises a dynamic gear 11, a paper-picking roller 10, and a paper feeding roller 12. The dynamic gear 11 is directly coupled to rotate the paper feeding roller 12. A clutch (not shown) comprised of a spring is inserted between the dynamic gear 11 and the paper-picking roller 10. The speed of the paper feeding roller 12 is designed to be greater than that of the paper-picking roller 10. When this mechanism is in action, the dynamic gear 11 drives the paper-picking roller 10 (the clutch is closed at this moment), so that the paper 13 is pulled by the paper-picking roller 10 and sent to the paper feeding roller 12. Since the paper feeding roller 12 has a higher speed than the paper-picking roller 10, the paper 13 drives the paper-picking roller 10 to reach a higher rotating speed than the dynamic gear 11. The clutch is thus in the open state without pulling the paper 13. The above-mentioned conventional structure can be seen in U.S. Pat. No. 5,630,580, “FEED ROLL ASSEMBLY ADAPTED FOR DELIVERING SHEET OF clutch still produces a pulling force on the paper 13 even when the clutching gap is insufficient. This is disturbing for the operations of processors.
U.S. Pat. No. 6,024,357, “DOCUMENT SHEET FEEDING MECHANISM” discloses a mechanical clutch and paper separating design. Its primary goal is to solve the problems of separating and pulling paper. It uses a mechanical clutch and an appropriate gap design so that the paper is not pulled in this case. With the paper separating design, there is a higher reliability in picking a single sheet of paper. Even so, the paper separating design and the paper-picking roller are disposed at the same position. Therefore, the paper separating effect is still not reliable in practice. Therefore, U.S. Pat. No. 6,540,220, “PAPER FEEDING SYSTEM WITH BOTH PAPER ENGAGING AND PAPER SEPARATING MECHANISMS” proposed an automatic document feeder. It uses a swing-arm paper-picking roller to touch and move the paper to the paper-separating roller. The paper-separating roller is provided with a friction board. When the paper-picking roller picks more than one sheet of paper and feeds them to the paper-separating roller, only the sheet that is in contact with the paper-separating roller is passed on while the sheet in contact with the friction board is left behind because the friction of the friction board is greater than the paper-separating roller. To prevent paper from pulling after entering the paper-separating roller or overfeeding, the swing arm is controlled by a motor to increase/decrease its speed or even reverse its rotating direction. Therefore, the paper-picking roller is lifted without contacting the paper, providing a gap between two sheets of paper.
Even though the above-mentioned proposals have the paper picking/separating functions, paper pulling still happens. Moreover, current processors often need to process recycled paper. Such paper makes the paper picking/separating even more difficult.